This invention relates generally to industrial combustion systems, and more particularly to methods and systems for reducing NOx in industrial combustion systems.
During the combustion of natural gas and pulverized coal, nitrogen oxides (“NOx”) emissions are formed by the oxidation of nitrogen in combustion air that is under high temperatures. At least some known NOx emission sources include devices such as, but not limited to, industrial boilers and furnaces, larger utility boilers and furnaces, gas turbine engines, steam generators, and other combustion systems. Because of stringent emission control standards, it is desirable to control NOx emissions by either suppressing NOx formation and/or by reducing NOx to molecular nitrogen (“N2”) and water (“H2O”).
At least some known combustion systems attempt to reduce NOx emissions from a furnace/boiler in at least the following stages: (1) before combustion—using pre-combustion control technologies, (2) during combustion—using combustion modification control technologies that modify the combustion process so that the combustion process produces less NOx, and/or (3) after combustion—using post-combustion control technologies that inject a selective reagent such as, but not limited to, ammonia (“NH3”), urea, and/or similar reducing agents, into the combustion flue gas to facilitate reducing NOx emissions.
Before combustion, at least some known pre-combustion control technologies burn low nitrogen fuels to facilitate reducing NOx emissions. However, generally pre-combustion technologies may be limited in reducing NOx emissions because air containing N2 is used to burn the low nitrogen fuel, and as such, oxidation of the N2 in the air may occur during combustion to form additional NOx emissions.
During combustion, at least some known combustion modification control technologies may reduce NOx by attempting to: (1) lower the temperature in a main combustion zone to suppress formation of NOx, (2) decrease the oxygen concentration in high temperature zones by supplying only enough oxygen to oxidize the fuel, but not enough to form NOx and carbon monoxide (“CO”) emissions, and/or (3) create conditions under which NOx can be reduced to N2 through reacting with hydrocarbon fragments. However, generally combustion modification control technologies include limited NOx emissions reduction, stringent operating tolerances, and limited residence times to complete combustion.
After combustion, at least some known post-combustion control technologies such as, but not limited to, Selective Catalytic Reduction (“SCR”) and Selective Non-Catalytic Reduction (“SNCR”) may be used to selectively reduce NOx emissions. In combustion systems using SCR technology, NOx is selectively reduced by injecting a nitrogenous reducing agent (“N-agent”) such as, NH3 or urea, into the furnace/boiler in the presence of at least one catalyst. Although the SCR system significantly reduces NOx more efficiently than known combustion modification control technologies, known SCR systems require a large catalyst bed, large amounts of catalysts, and catalysts disposal systems, all of which may be more difficult and more expensive to operate than combustion modification systems.
In combustion systems using SNCR technology, an N-agent is injected into the combustion flue gas at a high temperature. Under a non-catalytic reaction, the NOx formed during combustion may be reduced to N2 through a reaction with the N-agent. Although the SNCR system significantly reduces NOx more efficiently than known combustion modification control technologies, known SNCR systems reduce NOx less efficiently than the SCR systems. On the other hand, the SNCR system is generally less expensive than the SCR system, but more expensive than combustion modification systems. Moreover, although known SCR and SNCR systems reduce NOx more efficiently than combustion modification systems, both the SCR and SNCR systems include additional components that increase the overall costs, complexity, “foot print” (space in plant occupied by emissions control systems that could be devoted to production) and maintenance in comparison to known combustion modification control technologies.